Electrical filter structures are used in many applications. For example, electrical filter structures may be implemented to act as a low-pass filter, as a bandpass filter or as a high-pass filter. In the following, a brief introduction will be given to the design of filters.
FIG. 6a shows a schematic of a conventional lumped N-order low-pass filter (also briefly designated with LPF). The filter 600 is placed between a source 610 (modeled by a voltage source having a generator voltage VG and a resistance having a generator resistance RG) and a load 620 (modeled by a resistor having an impedance RL). Usually, the internal impedance (modeled here by the resistor having the generator resistance RG) and the load impedance (modeled by the resistor having the load resistance RL) are purely resistive. This justifies why FIGS. 6a and 6b represent them as resistors RG and RL. Moreover, the load impedance RL and the source impedance RG are typically coincident (the only relevant exception is the even-order Chebyshev filter) and equal to 50Ω in most cases.
The filter 600 itself consists of floor(N/2) series inductors L1, L3, . . . , LN and ceil(N/2) shunt series LC cells L2-C2, L4-C4, . . . , LN−1-CN−1. By definition, given a real number x, the function floor(x) returns the smallest integer greater or equal to x, while the function ceil(x) returns the greatest integer smaller or equal to x. More precisely, the inductors of the above mentioned shunt cells are short-circuited in all-poles types of filters, such as Butterworth, Chebyshev, and Bessel.
FIG. 6b shows a schematic of a so-called semi-lumped realization of the (low-pass) filter of FIG. 6a: all the inductors are realized with transmission line segments (also designated as transmission line portions) having (comparatively) high characteristic impedance, and all capacitors are realized with transmission line segments (also designated as transmission line portions) having (comparatively) low characteristic impedance. Herein, the qualifications “high” and “low” denote values which are much greater and much smaller (for example, by a factor of at least 1.5 greater or smaller, but advantageously by a factor of at least two or even and least 3 greater or smaller) than the working impedance (also designated as “internal impedance”) of the filter.
However, it has been found that the implementation of the filter structure 600a according to FIG. 6b with good filter characteristics is problematic in some technologies. For example, it has been found that it is sometimes difficult to obtain good broadband characteristics in a real implementation of the filter structure 600a. 
Also, it has been found that it is often desirable to implement a plurality of similar filters on a limited area while still maintaining a good isolation between the filters.
In view of this situation, it is an object of the present invention to create an electrical filter structure which brings along a good trade-off between filter characteristics and implementation area requirements.